MPG VS MPH

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grashley

grashley

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I made a 150 mile one way trip today in the truck.  Going north, I drove 70 mph (cruise) and coming south I drove 65 mph.  Same route, 90+ % interstate.  Same stale (5 month old) fuel for the entire trip. I don't drive the truck much.

2013 Ford F350 SRW  6.7L diesel

70 mph  =  16.6 mpg.  DECENT!

65 mph  =  18.0 mpg  WOW!

Just reporting for what it's worth.

 
Gordon I consider single tank mpg numbers meaningless. You can't fill a gas tank to the exact same level every time. It shuts off due to pressure which can change as temperate changes. A small head wind or a small tail wind will alter the numbers. The ambient temperature will affect the numbers too. But it is an interesting experiment.
 
My observation is:  When one of those goes up, the other goes down.  Nearly always.  Like was said - lots of variables to be considered.
 
The numbers are from the built in computer which works by magic, but agrees well with real world cross checks.
 
grashley said:
The numbers are from the built in computer which works by magic, but agrees well with real world cross checks.
Click to expand...
That doesn't matter, it is still impossible to fill a tank to the same level every fill up.
 
SeilerBird said:
That doesn't matter, it is still impossible to fill a tank to the same level every fill up.
Click to expand...

Tom the way I read his post, he went round trip on the same tank of fuel.  Did not fuel up. He said "Same stale (5 month old) fuel for the entire trip".
I think the difference is because he was going uphill going north (used more fuel), and downhill going south (used less fuel).  :eek: ::) :-\ ;) 
 
One good hill, or one Banzai pass, or any number of variables  could count for it all.
 
The OBD2 scanner I have uses the speedometer mileage and the fuel injection info to give me the instant MPG and then keeps track to give me the average MPG for the trip.

I can watch the MPG change as I speed up or slow down. As I get into headwinds, tailwinds, and sidewinds, I can see the difference.

There is a definite difference in driving 65 MPH vs. 75 MPH.
 
Rene T said:
Tom the way I read his post, he went round trip on the same tank of fuel.  Did not fuel up. He said "Same stale (5 month old) fuel for the entire trip".
I think the difference is because he was going uphill going north (used more fuel), and downhill going south (used less fuel).  :eek: ::) :-\ ;)
Click to expand...

I agree.  ^^^

However theoretically he had less fuel on-board on the second leg of the trip so less weight to carry around.
Also tires usually can change a pound or two when really warmed up or cooled and that can effect mileage somewhat.

If he stopped - and then restarted to return, the engine was warm and no warm up or high idle occurred.

Also just because he traveled on the expressway - I have seen the northbound and southbound lanes with very
different terrains - as they are rarely identical - not to mention the wind is changing all the time and can make a difference too. 
Passing traffic or you passing other traffic can also make a difference.

1.4 mpg for 5mph difference does seem kinda high to me.  But there is no doubt that driving slower will almost always give
better mileage numbers in most cases. 
 
It is a small sampling, I agree. But it is simple physics the faster you go the faster the gas gage goes down. That is with everything equal with no difference in winds terrain or even road surface. (Who knew road surface could effect mileage)
Bill
 
WILDEBILL308 said:
It is a small sampling, I agree. But it is simple physics the faster you go the faster the gas gage goes down. That is with everything equal with no difference in winds terrain or even road surface. (Who knew road surface could effect mileage)
Bill
Click to expand...
I did. Concrete is much smoother than asphalt and heavily repaired roads will also affect the mileage. There are just simply too many variables in computing gas mileage so I never pay attention to one tank figures other than to laugh.
 
SeilerBird said:
You can't fill a gas tank to the exact same level every time. It shuts off due to pressure which can change as temperate changes. A
Click to expand...
Yes you can in the Ford. Takes some work but you can get in an extra 3 gal or so by filling all the way up to the filler neck. ;)
 
70 mph  =  16.6 mpg.  DECENT!

65 mph  =  18.0 mpg  WOW!
Click to expand...

Not too surprising -- the drag increases as the square of the speed. But, as mentioned in other posts above, one way might average more uphill than the other way, and winds can be a factor. So it may be mostly speed (drag) change, or there may be other factors affecting the difference.
 
BruceinFL said:
Yes you can in the Ford. Takes some work but you can get in an extra 3 gal or so by filling all the way up to the filler neck. ;)
Click to expand...
But there is no way of knowing if there is an air bubble in the tank which can occur when you are parked at a slight angle.

Also you have the problem that no road is flat. Travelling between two cities and one will be at a higher elevation than the other meaning one trip is essentially going uphill and the return would be going down hill.
 
My mileage for the last trip to the North East of about 4500 miles was 9.1. I forgot how many tanks but enough to give a good number. The fun part is the trip last spring going West in lots of wind was 7.6. That was from Fort Worth to Phoenix, to Grand Canyon then back to Amarillo then home. There were tanks over 10 and some around 5.  ;)
Bill
 
My old silverado has an "average MPG" indicator.
When I first got the truck, the engineer in me went wild....
    I collected data  ;D

whenever I could drive a fixed constant speed for at least a few miles, I would reset that indicator and let it give me an average....with the cruise control set.
I forget how far now, but it would generally settle out fairly quickly...after a mile or so.... but I was able to get lots of data points at lots of different speeds..... for example at 45MPH over just a mile or two and up to fairly long hauls over 10 miles in some cases.... and lots of data points on different days, different roads, etc...so I could average out some measurement errors.

It made for an interesting curve....  If my memory serves, MPG peaked somewhere around 55 MPG (give or take 1 or 3, my memory is a bit fuzzy)
I was surprised, thinking that it would peak at a much lower speed.

It would be interesting to plot a similar curve for the motorhome....
 
blw2 said:
My old silverado has an "average MPG" indicator.
When I first got the truck, the engineer in me went wild....
    I collected data  ;D

whenever I could drive a fixed constant speed for at least a few miles, I would reset that indicator and let it give me an average....with the cruise control set.
I forget how far now, but it would generally settle out fairly quickly...after a mile or so.... but I was able to get lots of data points at lots of different speeds..... for example at 45MPH over just a mile or two and up to fairly long hauls over 10 miles in some cases.... and lots of data points on different days, different roads, etc...so I could average out some measurement errors.

It made for an interesting curve....  If my memory serves, MPG peaked somewhere around 55 MPG (give or take 1 or 3, my memory is a bit fuzzy)
I was surprised, thinking that it would peak at a much lower speed.

It would be interesting to plot a similar curve for the motorhome....
Click to expand...
I did the same thing and with my Veloster it seemed to do best around 50 mph.  But the outside temperature, the grade of the road and the time of day had an effect also.
 
Reset it at the end top of a long hill, watch the average soar to 99 MPG. Then take a photo to impress gullible friends!
 
A little off the topic, but pulling my big 5er I will average 11.5 to 11.8.  A couple of weeks ago I pulled a smaller trailer with my JD Gator on it and only got .5 mpg better mileage.  That really surprised me due to the weight difference and wind drag.
 
kjansen said:
A little off the topic, but pulling my big 5er I will average 11.5 to 11.8.  A couple of weeks ago I pulled a smaller trailer with my JD Gator on it and only got .5 mpg better mileage.  That really surprised me due to the weight difference and wind drag.
Click to expand...
Weight really doesn't have much of an effect on mileage. And the wind drag is determined by the front of the motorhome, not the trailer.
 
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